This invention relates to high purity high surface area alpha crystalline silicon nitride having a surface area of greater than about 25 m.sup.2 /g.
Silicon nitride is a refractory fibrous material. Depending on the length of the fibers and how they are processed, the material can be used in cutting tools, engines, reinforcing cast metal parts, or, by using the longer fibers, it can be made into fibers, felts for furnace walls and roof insulation, and flame curtains.
There are several known methods for producing silicon nitride, such as:
1. The reaction between elemental silicon and nitrogen;
2. The reaction between SiCl.sub.4 and anhydrous ammonia; and
3. The reaction of SiO.sub.2 with carbon in the presence of nitrogen.
A disadvantage of reaction 1 is that pure silicon is expensive and it also requires grinding to powder before it will react with the nitrogen. Grinding introduces impurities, making the process impractical if a pure product is required. In 2, the reaction between SiCl.sub.4 and ammonia produces products which are both difficult and expensive to remove. During the process, ammonium chloride is produced which must be sublimed off without losing the silicon nitride. This is difficult to do because the silicon nitride is so fine that it is carried out with the ammonium chloride. Furthermore, the process is difficult to control because any oxygen present causes oxidation to SiO.sub.2. In 3 the carbon source has been supplied in powder form. With the carbon in powder form, it is difficult to obtain a uniform mixture with the SiO.sub.2. Furthermore, some powder sources of carbon are not pure, and the result is relatively impure silicon nitride.
U.S. Pat. Nos. 4,264,565, 4,122,152, 4,368,180, 4,117,095, and 4,122,155 disclose processes for producing silicon nitride. Although U.S. Pat. No. 4,122,155 dicsloses silicon nitride of high surface area, it does not teach the high surface area values of the present invention for alpha crystalline silicon nitride.